This invention relates generally to the detection of energy losses in a steam system and is particularly directed to a steam trap monitor for detecting leaks in a steam trap.
Energy is lost from steam systems in various ways such as conduction, convection, and radiation. Additional losses may occur becuase of leaks such as in the joints of the piping system or because of faulty or permanently open bypasses. Some of the losses, such as pipe ruptures, blown gaskets, and other failures of pressure system integrity, are easily detected. Other losses are more difficult to detect and frequently go unnoticed.
One source of steam loss difficult to detect is that of the failed steam trap. Steam traps are incorporated in a steam system to remove condensate and air from the system while retaining live steam for use in a process. The condensate removed by a steam trap is returned to the heating source for re-heating to be again converted into steam. Traps can fail in either the open or closed position. Failure in the closed position is more likely to be noticed because it usually interrupts the process. Failure of a trap in the open position results in an abnormal amount of heat loss from the system, and may also cause a pressure loss affecting other parts of the steam system. Estimates of the number of failed steam traps, based on plant surveys, vary between 20 and 60 percent. Indeed, one expert maintains that after three years all steam traps are suspect and are very likely to have failed because of wear or contamination.
The use of an ultrasonic listening device is the most commonly used method for determining if a trap is working correctly. This method involves listening to a steam trap during operation to determine if it is working correctly. Each of the several types of steam traps exhibits a distinct sound. However, it is extremely difficult to detect leakage from a trap which is still operative. In addition, certain types of traps, e.g., the float-type steam trap, are frequently in a steady state mode rendering it extremely difficult to acoustically detect a malfunction. However, the sound test method offers advantages over visual checking of the steam trap, particularly where direct visual observation of the discharge from a trap is not possible such as when the steam trap discharge is piped into a closed condensate return system. A third steam trap checking approach involving temperature measurements either upstream or downstream of or within the stream trap is generally considered to be totally unreliable and has resulted in the rejection of large numbers of perfectly good traps and the overlooking of probably an equal number of faulty steam traps left in service.
Increased energy costs in combination with the aforementioned limitations of existing steam trap checking methods have led to the introduction of mechanical devices for monitoring steam trap operation. One approach employs a sensing unit positioned upstream from the steam trap for sensing the normal buildup of condensation. The absence of condensate indicates steam trap failure in the open position. Another available detector, also mounted upsteam of the trap, includes an internal weir which lowers the level of condensate when steam is being discharged by the trap. The absence of condensate at the tip of a probe indicates that the trap has failed and should be removed from service, cleaned, or repaired. A third device, integral with the steam trap itself, senses dynamic flow conditions through the discharge port with trap discharge resulting in movement of a spool exposed to the flow stream causing magnetic displacement of an external indicator ring. Each of these three devices is illustrated and briefly described in an article entitled "Checking Steam Trap Operation", in Plant Engineering, File 3580, Feb. 12, 1987, pp. 38-42.
Saturated steam and water at the same pressure have the same temperature. Therefore, the use of the aforementioned prior art temperature sensing devices alone will not detect the presence of both phases (water and steam) downstream from a steam trap. In addition, these prior art temperature sensitive detectors only provide an indication of the temperature of the condensate transiting the steam trap.
The present invention overcomes the limitations of the prior art by providing a steam trap monitor for accurately measuring the quantity of energy passing a given point in the form of steam and condensate and comparing that value to the amount of energy which should be passing that point as condensate to provide an indication of the amount of energy being wasted in the steam system. Located downstream from the steam trap, the present invention is capable of detecting both phases (water and steam) and of accurately measuring the heat content of each.